Fire barriers, such as fire doors for buildings and the like, may include a brake which allows closure of the barrier in response to certain emergencies, such as a fire. Such a brake may be electrically connected to the electrical power supply of the building such that when the supply of electricity to the building is interrupted, as often happens during a fire, the brake releases the fire barrier.
A problem with such a brake is that any power interruption, however brief and for whatever reason, causes the brake to release the fire barrier, frequently resulting in closure thereof. It is well known that not all power failures are the result of an emergency. For example, in the absence of a fire, a power failure may be produced by a false alarm of a smoke detector or the like, by maintenance or repair of the electrical wiring in the building, by an electrical disturbance in the transmission lines leading to the building, or by an electrical storm. In the absence of a fire, a power outage need not and preferably does not produce closure of the fire barrier. Shortly after a fire barrier closes due to a power failure, it is usually necessary to reset the fire barrier to the position thereof just before the power outage. Opening and positioning a fire barrier is typically time-consuming and may also be arduous.
It is known to add a timer mechanism to the brake which, when the supply of electricity to the building is interrupted, prevents the brake from closing the fire barrier for a duration no longer than a predetermined time period, which is typically 30 seconds or less. If the power interruption exceeds the predetermined time period, in which event the power outage is assumed to be the result of a fire or other emergency warranting closure, the brake releases the fire barrier for effecting closure thereof.
If the power supply to the building is restored within the predetermined time period, then the timer mechanism is reset thereby avoiding unnecessary closure of the fire barrier. Resetting the timer mechanism entails returning the timer mechanism to the state from which the timer mechanism may automatically respond to a power interruption in the aforesaid advantageous manner. A known timer mechanism may, at the end of a power interruption which is less than the predetermined time period, automatically reset itself. The timer mechanism is then ready to respond to subsequent power failures.
Such prior art timer mechanisms suffer a drawback in that they must be manually reset so that the timer mechanism will respond to a subsequent power outage.
Accordingly, it is desirable to have a timer mechanism for a fire door which can be automatically reset following a power outage and which uses relatively few components so that the cost of the timer mechanism is affordable.